T-mobile refusing to give me my credits from a promotion I qualified for

In September 2023 I ported three lines from ATT to T-Mobile to take advantage of the get the "get the iPhone 15 on us" promotion on two of the lines. We traded in two eligible iPhones to qualify for the credit. The Yukon, OK store manager also promised an additional  20% "insider" discount that would lower my monthly bill from $150 to $120. After four full billing cycles I have yet to receive either the automatic bill credits for the new devices or the 20% discount as agreed. Each month I call and complain and, full disclosure, the customer service reps have twice provided one-time credits to my account in the correct amount to cover the monthly payments on the two phones. This month when I called I was told there would be no more one-time credits and I would just have to wait until T-Mobile decides to start the automatic credits. I now believe I this may never happen, let alone receive the promised 20% discount. I feel like they have no intension of honoring their agreement and want to play the long game, hoping I will admit defeat, go away and just pay the higher amount. I never thought I would admit it, but I wish I never switched from ATT.